Semiconductor technology development has been following two significant trends from its very beginning. One trend is that the operating frequency of an integrated circuit (IC) device gets higher and higher. For example, the main operating frequency of the Intel386™ central processing unit (CPU) developed in the mid-1980's was only about 33 MHz. In contrast, the main operating frequency of today's Intel Pentium4™ CPU reaches 3.2 GHz, which is approximately 100 times faster than Intel386™ CPU. Higher frequency makes it possible for a device to process more data within a limited time period.
The other trend is that the number of transistors per IC grows exponentially according to the well known Moore's Law. For instance, the Intel386™ CPU hosts only 275K transistors while the Intel Pentium 4 CPU hosts 42 million transistors. The placement of more and more transistors on a silicon chip of limited dimension continuously reduces the minimum feature size, i.e., the dimension of the smallest feature actually constructed on the chip in the manufacturing process. As a result, the minimum feature size of a device drops sharply from about 1.5 micron in the Intel386™ CPU to just 0.13 micron in the Intel Pentium 4 CPU. The higher volume of electrical components on a single silicon chip tremendously enhances an IC device's data-processing capability, which in turn requires better input/output (JO) and package protection.
The downside of the aforementioned technology revolution is that certain previously ignored issues that are related to either a device or an electrical package protecting the device are becoming so critical that they may substantially hinder the progress of semiconductor technology if not dealt with appropriately. Among them, the issue of crosstalk-related signal integrity is one that deserves special attention. Crosstalk refers to electrical noise caused by mutual inductance between two conductive lines, e.g., two conductive traces in a device or two package leads, due to their proximity. If such noise is great enough, it may corrupt the electrical signal traveling through a device and disturb the normal function of the device.
FIG. 1A schematically depicts the mutual inductance between two adjacent circuits 120 and 126. Circuit 120 includes a power source 122 that generates a current moving in a counter-clockwise direction as indicated by the arrows. Some electric energy generated by power source 122 is converted into magnetic energy and stored in the magnetic field surrounding the circuit path as indicated by the magnetic flux lines 124. The amount of magnetic energy stored per unit volume is proportional to the current in circuit 120. A portion of the magnetic flux passes through loop 125 formed by circuit 126, which has a resistor 128, but no power source. Faraday's Law states that a time-varying current in circuit 120 results in a time-varying magnetic field surrounding circuit 120, which subsequently induces a voltage and current in circuit 126. The direction of the induced current in circuit 126 is opposite to that in circuit 120 or clockwise. The induced voltage or crosstalk is proportional to the rate of change of current in circuit 120.
FIG. 1B illustrates how crosstalk may affect signal integrity in an integrated circuit. Curve 140 represents an ideal step response in which the voltage of a digital circuit jumps from Vmin (or zero) to Vmax instantaneously. Such an ideal scenario rarely exists in real life because it always takes some time for the voltage level to rise or drop from one extreme to another extreme. Curve 142 demonstrates a more realistic step response in which the time it takes for the voltage to rise to 70% of Vmax, or trigger voltage Vtrigger, is referred to as the rise time Tr, of the circuit. The rise time is an important parameter for measuring the speed of a circuit. Specifically, the shorter the rise time the faster the circuit. However, the mutual inductance between this and adjacent circuits may induce a voltage Vinductance in the circuit (curve 144). When voltage Vinductance that has a different polarity is superimposed on the step response (curve 142), the voltage may not reach the trigger voltage Vtrigger during the rise time indicated by curve 146, but only a lower voltage level Vdelta. Such a lower voltage level may cause the circuit to malfunction and produce undesirable results.
The magnitude of crosstalk caused by mutual inductance as shown in FIG. 1A is a function of the distance between the two neighboring circuits 120 and 126, their relative orientations, the overlapping area of the loops formed by the two circuits and the operating frequency in the circuits. Higher frequency usually results in more significant crosstalk. In an IC device, smaller minimum feature size reduces the distance between two adjacent circuits and thus increases the crosstalk in the device. As a result, both of the two development trends discussed above exacerbate crosstalk-related problems.
Meanwhile, the signal voltage of an IC device is roughly proportional to the device's minimum feature size. For example, the signal voltage of a device having a minimum feature size of 0.3 micron is about 3V while the signal voltage of a device having a minimum feature size of 0.13 micron is only about 1.3V. The trigger voltage correspondingly reduces from 2.1V to 0.9V when the minimum feature size reduces from 0.3 micron to 0.13 micron. While lower signal voltage is welcome because it reduces a device's power consumption, it unfortunately reduces the device's tolerance threshold for crosstalk at the same time. Therefore, the crosstalk that previously could be safely ignored may pose a significant challenge to a device's normal operation now and therefore should be treated appropriately.
In view of higher crosstalk due to higher operating frequency and smaller minimum feature size and lower tolerance of crosstalk by today's IC devices, it is highly desirable to develop a new strategy that can effectively reduce the crosstalk-related signal integrity issue without significant changes to the conventional semiconductor packages.